The invention relates to a cooling control system for an internal combustion engine equipped with a supercharger, which controls cooling of the engine, supercharger, etc. in an improved manner.
Some internal combustion engines for automotive vehicles have a turbocharger arranged in the intake passage thereof as a supercharger for enhancing the engine output.
In an engine equipped with this type of supercharger, the temperature of the turbocharger increases during operation thereof, which may cause thermal damage to engine components, such as seizure of bearings and carbonization of lubricating oil and hence degradation of its lubricity. A cooling control system for a water-cooled type turbocharger, which is intended to prevent the thermal damage, is conventionally known. The cooling control system is adapted to drive a pump to cause cooling water to circulate through the turbocharger during operation of the engine to thereby water-cool the turbocharger.
Further, a cooling control system of this type has been proposed, for example, by Japanese Provisional Utility Model Publication (Kokai) No. 59105029, in which a pump is driven for water-cooling the turbocharger for a predetermined time period after stoppage of the engine in order to prevent the thermal damage which tends to occur after stoppage of the engine since the turbocharger continues to rotate by inertia after stoppage of the engine to thereby rise in temperature.
However, the proposed cooling control system is constructed such that simultaneously when the ignition switch of the engine is turned off, the pump is started to operate and supply the turbocharger with cooling water over the predetermined time period immediately after stoppage of the engine. Therefore, the pump is driven for operation even when the turbocharger does not require cooling by water circulated by the pump.
More specifically, the thermal damage described above due to high &temperature of the turbocharger after stoppage of the engine tends to occur when the engine is stopped after being operated under a medium or high load. Therefore, when the engine has been stopped after being operated under a medium or high load, the necessity to cool the turbocharger by circulating the cooling water by the pump is high for positive prevention of the thermal damage. However, if the engine has been under a low load condition before stoppage of the engine, it is not very necessary to cool the turbocharger after stoppage of the engine since the temperature of the engine is not very high. Therefore, the above proposed system which cools the turbocharger through driving of the pump after stoppage of the engine may not always meet actual thermal conditions of the engine and turbocharger.
Further, according to the above proposed cooling control system, the control of cooling of the turbocharger is carried out while the engine is in stoppage, i.e. while the battery installed on the vehicle is not charged with electricity. Therefore, unnecessary driving of the pump results in wasteful consumption of the battery.
The temperature of the turbocharger on and after stoppage of the engine depends, on an operating condition in which the engine has been immediately before stoppage, such as load on the engine and ambient temperature, and may vary according to different cases. Therefore, even if the temperature of the turbocharger on stoppage of the engine is identical between the cases, the rise in the temperature of same thereafter can be different from each other.
In particular, the degree of rise in the temperature of the turbocharger after stoppage of the engine largely depends on the load on the engine applied immediately before stoppage thereof. That is, it is larger and hence is more liable to cause the thermal damage as the load on the engine immediately before stoppage is higher. Further, the temperature of the turbocharger on stoppage of the engine not only depends on the load on the engine immediately before stoppage but also on external factors such as air streams produced by running of the vehicle and cooling the turbocharger, capacity of the radiator fan, etc. to a large extent. The degree of dependency on the external factors is large particularly in the case where the temperature of the turbocharger is detected by the use of cooling water temperature, since the specific heat of cooling water is small.
However, in the aforesaid conventional cooling control system the pump is driven over the predetermined time period, depending on the temperature of the turbocharger on stoppage of the engine, the temperature of same being influenced by external factors as above. Specifically, when the detected temperature is higher than a predetermined value, the predetermined time period is set to a constant value, while when the detected temperature is lower than the predetermined value, the predetermined time period is set to 0, that is, the pump is not driven. Therefore, it is impossible to carry out cooling control in response to the load condition of the engine immediately before stoppage of the engine. Further, it is impossible to drive the pump to just a sufficient extent depending on different manners of rise in the temperature of the turbocharger after stoppage of the engine. If the predetermined time period is set to a longer time period for prevention of the thermal damage, in case that the temperature of the turbocharger rises to the maximum possible degree after stoppage of the engine, the pump will be driven for an excessively long time period even when the turbocharger temperature actually rises to a lower degree than expected, which will result in wasteful consumption of the battery. On the other hand, if the predetermined time period is set to a shorter value for prevention of such wasteful battery consumption, it is impossible to prevent the thermal damage when the turbocharger temperature actually rises to a higher degree.
Further, the aforesaid conventional cooling control system is not provided with a fail-safe function against malfunction of its component parts or related devices. If the pump goes out of order, for example, the turbocharger may suffer from seizure etc. i.e. inoperativeness of the turbocharger itself resulting from malfunction of the cooling system therefor.
In the meanwhile, an engine-room cooling control system has been conventionally proposed by Japanese Utility Model Publication (Kokoku) No. 55-34101, in which an electric fan is arranged in the engine room to be operated or stopped depending on the temperature of the engine room and that of the engine, so that the temperature of the engine room may be controlled to a proper value to thereby prevent thermal damage to the engine such as overheating of the engine and vapor locking of fuel.
However, this cooling control system has disadvantages such that if the engine is equipped with a turbocharger, the temperature of the turbocharger and that of the engine room cannot be properly controlled after stoppage of the engine.
More specifically, the conventional cooling control system is constructed such that the control of operation of the electric fan responsive to the engine room temperature etc. is carried out only during operation of the engine, so that it is impossible to prevent rise in the temperature of the turbocharger after stoppage of the engine, and hence to prevent thermal damage to the turbocharger. Further, since the engine room temperature also rises with rise in the temperature of the turbocharger which is a heat source in the engine room, it is impossible to prevent the above-described thermal damage to the engine.
A cooling control system is also known in which a control device is provided separately from a control device for controlling the engine itself, for controlling the operation of a cooling device which cools the engine, in response to an output of a sensor for detecting operating conditions of the engine whereby the engine is cooled to just a sufficient extent so as to prevent excessive rise in the engine temperature. A cooling control system of this type has been proposed by Japanese Provisional Patent Publication (Kokai) No. 57-191415 in which an electric fan is provided which operates in the event of malfunction of a driving device for a water pump as the cooling device or malfunction of the first-mentioned control device to air-cool the engine even during the malfuction to thereby prevent rise in the engine temperature.
However, in this cooling control system, the water pump and the electric fan ar intended for cooling the engine alone. Therefore, if the control system is applied to an engine equipped with a turbocharger, it is impossible to prevent excessive rise in the temperature of the turbocharger, and hence excessive rise in the temperature of the engine due to rise in the temperature of the turbocharger, which is one of main heat sources in the engine room, and therefore the proper performance or sufficient durability of the engine may not be obtained.